Metasomatic Evolution in Tectonically Mixed Zones (Mélange) and Significance for Geochemical Evolution of the Slab-Mantle Interface
Abstract
Fluid flow focused in highly deformed zones (shear zones), and the physical juxtaposition of chemically disparate rocks (via mechanical mixing) in such zones, can lead to extensive metasomatism, including volume strain, and result in rocks with hybridized compositions little resembling the compositions of the incorporated rock types [1-5]. In the Catalina Schist (California), lawsonite-albite, lawsonite-blueschist, and amphibolite-facies units contain shear zones at scales of meters to kilometers, each containing "blocks" (with more spherical or more tabular dimensions) co-facial in grade with the "matrix" surrounding these blocks [1-3]. Oxygen isotope data for these "mélange" units, and adjacent more "coherent" expanses, indicate enhanced fluid flow in the more strongly deforming mélange zones while fluid flow in coherent domains was dominantly fracture-controlled and episodic. The amphibolite-facies mélange unit shows evidence for km-scale equilibration of varying mineral assemblages with H2O-rich fluids with uniform O and H isotope compositions consistent with a lower-grade metasedimentary source. This unit is believed to have formed largely by mechanical mixing of mafic and ultramafic compositions, partly because of the scarcity of sedimentary blocks. However, the mélange matrix in this unit preserves a number of sedimentary chemical/isotopic characteristics (e.g., Pb isotope compositions [3]) that could reflect the incorporation of sedimentary rocks, with or without fluid-related fractionation, and possibly fluid-mediated additions. Tectonically mixed zones such as these, if volumetrically significant at the slab-mantle interface, could exert disproportionate control on the compositions of hydrous fluids or silicate melts emanating from subducting slabs and entering the forearc to backarc mantle wedge, including those contributing to arc magmatism [1-5]. Geochemical studies of arc lavas should consider the possibility that the "fluids" contributed from slabs to arc source regions bear chemical/isotopic signatures reflecting their interaction with these hybridized zones produced by mixing of varying proportions of sedimentary, mafic, and ultramafic compositions. Also, the high-variance hydrous mineral assemblages created by these coeval mechanical and metasomatic processes (e.g., nearly monomineralic chlorite, talc, and amphibole schists) could play an important role in the volatiles budgets at subduction zones (i.e., having stabilities to P and T significantly higher than those for mineral assemblages in metabasaltic and metasedimentary rocks containing the same mineral phases [1,4]). Field, petrologic/geochemical, theoretical, and geophysical studies should work toward assessment of the volumetric significance, physical properties, and devolatilization histories of these hybridized compositions. [1] Bebout and Barton (2002) Chem. Geol. 187:79-106 [2] King et al. (2006) Ear. Planet. Sci. Lett. 246:288-304 [3] King et al. (2007) Chem. Geol. 239:305-322 [4] Spandler et al. (2008) Contrib. Mineral. Petrol. 155:181-198 [5] Miller et al. (2009) Lithos 107:53-67
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V33F..05B
- Keywords:
-
- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 3653 MINERALOGY AND PETROLOGY / Fluid flow;
- 8021 STRUCTURAL GEOLOGY / Melanges